Cardio Workouts for Fat Loss: What the Science Says

“Practically speaking, exercise alone doesn’t reduce weight dependably,” wrote Donald G. Cooley in The New Way To Eat to Get Slim.

“Suppose you are of average weight and you take a brisk two-mile hike on level ground. You figure pridefully that you have burned up a few ounces of fat. The sad truth is that your two-mile hike consumes an excess of a mere 115 calories, which are easily replaced by a mild snack consisting of three graham crackers.”

He wrote that in 1941.

Since then, many hundreds of studies have been published, the vast majority of which have come to much the same conclusion.

Cardio, by itself, is not a very effective way to promote fat loss.

Back in 1997, Dr Wayne Miller and colleagues at The George Washington University Medical Center set out to determine if adding aerobic exercise to a low-calorie diet accelerates weight loss [1].

Miller and his associates examined 493 studies carried out between 1969 and 1994. What they found was that diet and exercise provides only a very marginal benefit when compared to diet alone.

The average amount of weight lost after a 15-week program of regular aerobic exercise was seven pounds. Over the same period, dieting cut weight by roughly 17 pounds. When exercise and diet were combined, average weight loss was 20 pounds — just three pounds more than diet alone.

A 2011 review that looked at 14 studies on aerobic exercise and weight loss also shows less than stellar results, concluding that the value of aerobic exercise as an “independent weight loss intervention for overweight and obese populations is limited.” [2].

As part of the HERITAGE Family Study, one of the largest training studies of its kind, researchers followed a large group of 557 men and women as they embarked on a 20-week exercise program [3].

Each subject was required to exercise three times per week for an average of 42 minutes. Researchers even went to the trouble of having each bout of exercise monitored by an exercise technician and a computer.

Following a grand total of 60 exercise sessions over a period of almost six months, the average amount of fat lost was slightly less than two pounds.

The video below does a great job of summarizing the science behind why cardio workouts aren’t a particularly effective way to lose weight.

But there are two sides to any story, and this one is no different. Before you force cardio exercise into a small cupboard under the stairs, lock the door and throw away the key, here are some things to think about.

There are many different types of cardio workouts out there, ranging in intensity from low to medium to high.

You’ve got low-intensity steady-state cardio (e.g. walking or an easy bike ride), moderate-intensity exercise (e.g. jogging, rowing, or the stair climber) or high-intensity cardio (e.g. high-intensity interval training).

Putting them all in the same category ignores the fact that 30 minutes of coasting along on the elliptical machine while watching TV and texting requires a lot less energy than the same amount of time spent cycling up Mount Ventoux as fast as your legs will carry you.

Both are forms of steady-state cardio. But the latter is going to have a much greater physical impact than the former.

Most studies out there have looked at low-intensity or moderate-intensity cardio, which in and of itself has only a modest impact on energy expenditure and weight loss. High-intensity cardio is another story entirely.

As an example, researchers from Appalachian State University found that 45 minutes of high-intensity steady-state cardio (cycling at 85% of your maximum heart rate) burned over 700 calories – 519 during the workout itself and 190 after it had finished [4]. And that was net rather than gross calorie expenditure.

What’s the difference between net and gross energy expenditure?

Let’s say that you jump on the bike in the gym. When you get off, the digital readout says that you’ve burned 500 calories. But those aren’t necessarily 500 extra calories.

Why not? If you’d spent that time sitting in front of the TV doing nothing, your body would have been burning calories anyway.

Gross calorie expenditure refers to the number of calories you burn during exercise plus resting energy expenditure for an equivalent time. Net calorie expenditure refers only to the extra number of calories you burn during exercise.

How much of a difference is there between the two numbers?

In one study, the gross number of calories burned during 45 minutes of aerobic exercise was 255 calories. But the net figure, which represents the “real” number of extra calories burned, was just 187 calories [5].

You also need to take into account the physical characteristics of the people taking part in the study.

In one 12-week trial, a group of obese men who did 60 minutes of moderate-intensity exercise (brisk walking or light jogging) every day lost an average of 13 pounds of fat [6].

In addition, most of the lost fat came from subcutaneous (under the skin) stores. Considering all they were doing was walking/light jogging, losing just over one pound of fat per week is a decent result.

Although this is a lot more than some of the other studies we’ve looked at it, the people taking part in this trial trained for more than one hour, every single day.

They were also obese, with an average body mass index (BMI) of 31. They’re carrying so much extra weight that just going for a walk expends a massive amount of energy.

Here’s something else that’s very important.

Most studies report only the average results for a group of people. But this can mask large differences in individual results.

When some people increase the amount of exercise they do, they get very hungry. This leads them to eat more, which ends up reducing the calorie deficit created by exercise.

Researchers have labeled them compensators, because they compensate for the calories burned during exercise by eating more.

Take a look at the figure below, which comes from a study looking at the variability in weight loss following 12 weeks of supervised exercise [7]. It shows individual changes in both weight (BW) and fat mass (BF).

Don’t pay too much attention to the changes in body fat. It was measured using BIA (the technology used in body fat scales), which is not a reliable way to track changes in body composition over time.

When the data from all the subjects was pooled, the average drop in weight was eight pounds.

However, there was a large variability in the amount of weight lost. One subject lost 32 pounds. But another subject taking part in the exact same study gained almost four pounds.

The non-compensators – that is, the people who didn’t compensate for the calories burned during exercise by eating more – reduced their calorie intake by an average of 131 calories per day. But the compensators ended up eating more, increasing their calorie intake by an average of 268 calories per day.

Many studies show that cardio produces little in the way of fat loss. But that doesn’t mean everyone in the study got the same results. Weight loss in one set of subjects may simply have been “cancelled out” by weight gain in another

To give you an idea how exercise can vary in its effectiveness as a tool for losing weight, I’m going to compare two people, both of whom are doing exactly the same thing in the gym.

The first is a 250-pound overweight male. He’s a non-compensator, and doesn’t get extremely hungry when he does more exercise.

The second is a 150-pound overweight female, who tends to compensate for the calories burned during exercise by eating more. She gets very hungry whenever the amount of exercise she does exceeds a certain threshold. Keeping her food intake under control is a constant struggle.

They both go to the gym 4-5 days a week, for around 45 minutes. Most of that time is spent walking on a treadmill set on a slight incline.

The male has one big advantage that makes exercise a lot more effective where weight loss is concerned. Namely, that he’s carrying around an extra 100 pounds.

Simply going for a walk burns off a large number of calories. It’s the equivalent of someone with a normal BMI walking on the treadmill with a heavy rucksack strapped to their back.

What’s more, he doesn’t find that exercise makes him significantly hungrier than before. So he’s able to create a much larger energy deficit, which in turn means that weight is lost more quickly.

Our 150-pound overweight female burns fewer calories during exercise. She also ends up either partially or completely compensating for the calories burned during a workout by eating more food. As a result, her rate of fat loss will be significantly slower.

So even though we have two people, both with the same goals, and both following the same exercise routine, the potential exists for them to experience very different results.

What About Strength Training and Fat Loss?

Strength training is often touted as a metabolism-boosting marvel that will help you burn calories around the clock, while you’re sleeping, sitting at your desk or resting on the couch.

Cardio might burn more calories during the workout itself, the theory goes, while strength training helps you build muscle so that you’ll burn more calories 24/7.

While it’s true that muscle has a higher resting metabolic rate than fat, the overall difference between the two is relatively small. 

In fact, research shows that one pound of muscle burns around 6 calories per day at rest, compared to 2 calories per pound of fat tissue.

You’d need to gain a huge amount of muscle for it to have a meaningful impact on your resting metabolic rate, far more than the twenty or thirty pounds of muscle the average person is likely to gain over the course of their training lifetime.

However, that doesn’t mean resistance training is pointless if fat loss is your goal.

In fact, a resistance training program that works all the major muscle groups will improve your body composition in a number of different ways.

Firstly, strength training burns calories, and makes a direct contribution to the calorie deficit required to lose fat.

Second, if you don’t do some kind of resistance training while you’re dieting, a lot of the weight you lose will come from muscle as well as body fat.